Just loaded some 300wsm Norma brass. After firing the brass shrunk 10 thousands. Any one know the reason? Please help! Even after neck sizing the brass was still 6 to 8 thousands shorter than before firing.

Brass gets shorter when it is fired. The case expands, causing brass flow. When it is sized, it will get longer.

But the brass was still shorter after neck sizing than before it was fired. Is this common? What would the cause be? An over sized chamber causing excessive flow?

What win 69 is saying is that when you fire the round, the case expands in diameter and gets shorter in length as a result. As mentioned, if you were to full length resize a lot of your length will come back. Anyway, most all brass does this to some extent so as long as everything else is normal you do not need to worry about it.

As posted, the brass has blown out laterally to fit your chamber (it is now fatter) and so it is shorter. When the brass came from the factory it was generic --one size fits all -- now it only has to fit one chamber, yours. So it is not to worry. This is also why you shouldn't trim your brass to a uniform length until it is fire formed in your chamber.